Powder coating system for difficult to handle powders

ABSTRACT

A powder coating system for difficult to handle powders includes a powder hopper containing fluidized powder particles, a pump body connected to the hopper and having an inlet in communication with the hopper, an ejector nozzle mounted to the pump body and aimed at an outlet thereof, a second nozzle mounted to the pump body and aimed at the inlet thereof and a powder application device for receiving powder particles from the outlet of the pump body and spraying them onto an article to be coated. The ejector nozzle sprays pressurized air toward the outlet to transport powder particles along a flow path which extends from the powder hopper into the pump body via the inlet and then out of the pump body via the outlet. The second nozzle sprays air pulses at the inlet in a direction opposite the particle flow along the flow path caused by the ejector nozzle. This spray from the second nozzle creates microvibrations in the inlet of the pump body to eliminate powder particle adherence and cohesion thereat. The pump body may have separate, spaced intake and ejector portions, with the ejector nozzle mounted in the ejection portion and the second nozzle mounted on the intake portion.

FIELD OF THE INVENTION

This invention relates to an improved method and apparatus for the gastransport of powder, and more particularly, to methods and apparatuswhich assure delivery of uniform and consistent quantities of powder perunit time to a spray gun for application to the surface of an article tobe coated.

BACKGROUND OF THE INVENTION

In a typical powder coating apparatus, powder is maintained in afluidized state within a hopper having a fluidized bed, and istransported from the hopper to a pump chamber of a powder pump,whereupon an ejector nozzle in the pump directs a high pressure gasstream toward an outlet of the chamber and along an exit tube connectedto the chamber. As a result, the powder is conveyed through the exittube to a spray gun which sprays the powder toward the surface of anarticle to be coated. The ejector nozzle must be operated atsufficiently high pressure to transport the particles to the end of theexit tube and through the spray gun. During this pumping operation, thehigh pressure oft he ejector nozzle creates a relatively low pressure atthe intake to the pump chamber, thereby drawing powder particles fromthe powder container and into the chamber for ejection therefrom alongthe exit tube.

One typical application for an apparatus of this type involves coatingthe surface of an aluminum joint with powdered solder flux prior tosteps of heating and melting the solder flux to weld the connectingaluminum elements and form the joint. The reliability of the connectiondepends upon the uniformity of the flux coating.

In these and other applications wherein powder particles aregas-transported to coat a surface, including applications which involveelectrostatic charging of the particles, problems may result due toadherence and accumulation of the powder particles at the inlet of thepump from the hopper. This occurs with powders that have the property ofrelatively easy coherence, easy adherence due to viscosity, relativelylow slipperiness, or powders which simply do not have good flowcharacteristics and tend to agglomerate. Powdered solder flux is such apowder. With these types of powders, even powder flow from the hopper tothe spray gun is difficult to achieve, and therefore, the amount ofpowder transported per unit time fluctuates. As a result, it isdifficult when spraying these powders to achieve a uniformly thickcoating of powder on the article and to consistently apply a uniformcoating from one article to the next in a production line situation.This is possibly due to changes in the flow of the powder resulting froma reduction in the cross sectional area at the inlet to the pump orbecause of the influence of static electricity among the particles dueto friction which encourages agglomeration of the powder particles.

It is therefore one objective of the invention to achieve greateruniformity in powder transport per unit time by minimizing or reducingaccumulation and adherence of the particles during transport from thepowder hopper to the article to be coated.

In other applications for powder coating via gas transport, it is oftennecessary to intermittently turn the powder pump on and off. One examplefor the need of this type of operation involves coating articles carriedon a conveyor, wherein it is desirable to spray coat the articles at acoating station on the conveyor, and then turn the pump off until theconveyor moves the next article to the coating station. For applicationswhich require ON/OFF operation of a powder coating apparatus, it isdesirable to achieve precise control of the powder flow to effectivelyturn the apparatus off and on at the desired times. Otherwise, powder iswasted. It is also desirable to spray the same quantity of powder oneach article.

In the past, pinch valves made of rubber tubes have been used to controlintermittent powder flow at a powder hopper. Rubberized pinch valves arepneumatically operated and are relatively simple and inexpensive.However, during closing, these valves have a tendency to close upon somepowder particles. Eventually this causes gaps between the opposingrubber portions and produces air leaks. These leaks allow some powder tomove to the downstream side of the pinch valve. Because there is no wayof knowing how much powder has reached the downstream side of the pinchvalve, the amount of powder ejected during each ON/OFF cycle may vary.This will produce nonuniformity in coating an article. Additionally,after a certain number of switching operations, the rubber of the pinchvalve becomes fatigued and it deteriorates to a point where it isimpossible to use. Again, as this occurs, the effectiveness of the valvebecomes questionable. As varying quantities of powder reach thedownstream side, the apparatus will eject varying quantities of powderduring each ON/OFF cycle.

It is another objective of this invention to more precisely control theON/OFF switching operation of a powder pump, thereby to assure deliveryof uniform powder quantities during each ON/OFF cycle of operation.

It is still another objective of the invention to simultaneously achieveuniform powder ejection per unit time during an ON cycle and toeffectively stop and start powder ejection during switching operationbetween ON/OFF and OFF/ON, respectively.

It is still another objective of the invention to minimize the adverseeffects of powder cohesion, adherence, agglomeration and friction duringflow from a fluidized powder hopper to an outlet end of a spray gun,thereby to achieve improved uniformity in powder delivery to an articleto be coated and to produce a more stable coating thereon.

SUMMARY OF THE INVENTION

The above-stated objectives related to improved uniformity in powderdelivery during operation are achieved by directing low pressure gaspulses toward the fluidized powder hopper, counter to the normal flowdirection of the powder out of the hopper, thereby to createmicrovibrations within the powder intake portion of the ejector pumpbody and eliminate adherence and cohesion among powder particles at thepump inlet. These low pressure gas pulses are produced during the ONcycle of the pump.

The above-stated objectives related to improved control over the ON/OFFswitching operation for a powder pump are achieved by directing areverse flow of gas through the powder pump inlet toward the fluidizedhopper and counter to the normal flow direction of the powder, at apressure sufficient to prevent particles from entering the pump inletduring the OFF cycle. For both the low pressure reverse direction pulsesprovided during the ON cycle, and higher pressure reverse air flowprovided during the OFF cycle, the nozzle or nozzles which create theseair flows are oriented such that the axes of these spray nozzles do notintersect the axis of the ejector nozzle, and the flow paths of the airflows produced by these nozzles do not cross with the flow path of theejector nozzle air flow.

According to a first preferred embodiment of the invention, a powderejection apparatus includes a fluidized powder container, a pump bodywith a pump inlet in communication with the powder container and anoutlet for directing the powder to an article to be coated, an ejectornozzle for directing high pressure air through the pump body toward theoutlet and a reverse pulse nozzle directed at the pump inlet.Preferably, the nozzles are oriented such that their spray paths do notintersect.

When the ejector nozzle is operated at a relatively high pressure,powder is transported from the pump body through the outlet. Lowpressure at the powder intake portion created by the ejector nozzlecauses powder to flow therethrough from the powder hopper for subsequentejection. While the ejector nozzle operates, the reverse pulsing nozzlealso operates at a relatively low pressure, and is pulsed off and onintermittently at high frequency to cause microvibrations of powderparticles within the powder intake portion. This prevents adherence andaccumulation of powder particles to the side walls of the powder intakeportion, and thereby produces a uniformity in powder flow through theapparatus per unit time. As a result, more uniform coating of an articlemay be achieved.

According to a second preferred embodiment of the invention, moreprecise control of the switching operation between OFF and ON, and viceversa, is achieved via the use of a reverse flow nozzle adapted to spraypowder back into the powder hopper when the ejector nozzle is off. Whenthe ejector nozzle switches back on, the reverse flow nozzle turns off.Each time the ejector nozzle is turned on, the powder must traverse theentire flow path from the powder hopper to the outlet of the powderejection apparatus, because the reverse flow nozzle had previously blownall the powder within the hopper outlet back into the powder hopper.This assures consistency in powder delivery during the ON cycle of theapparatus.

This embodiment of the invention represents an improvement over priorpinch valves located between the hopper and the pump, which are subjectto wear over a period of time and inconsistent performance, due toleakage of powder therethrough. With this embodiment, no powder residesin the pump body prior to turning the ejection nozzle back on.

In one variation of the second preferred embodiment of the invention,the ejector nozzle and reverse flow nozzle are mounted to separatebodies which are interconnected via a connector line. In anothervariation, the ejector nozzle and the reverse flow nozzle are mounted tothe same body and oriented such that their spray paths do not intersect.

A third preferred embodiment of the invention provides the advantages ofboth the first and second embodiments by utilizing a single nozzle toprovide reverse pulsing during the pump ON cycle and reverse flow toswitch the apparatus OFF at the end of the ON cycle. This nozzle is fedby two separate fluid lines, each with a separate solenoid valve.Operation of the valves is controlled by an electrical controller whichalso controls operation of the flow of fluidizing air for the powderhopper, ejector air for the ejector nozzle and transport air injected bya transport nozzle located downstream of the ejector nozzle. Powderdelivered by this embodiment of the powder ejection apparatus is carriedby an air stream which comprises the confluence of the air from theejector nozzle and the air from the transport nozzle.

With this embodiment, precise switching is achieved to more effectivelyturn the apparatus OFF and ON when desired, and vice versa, and in amanner which assures consistent delivery of powder at the beginning ofthe next ON cycle. Additionally, due to the use of pulsing operationduring powder ejection, uniformity in powder delivery is achieved duringthe ON cycle. Preferably, the controller also controls delivery ofpowder to the hopper, as needed, and controls rotation of a stirringblade within the hopper to minimize channeling of powder, or theformation of chimneys of air in the powder within the hopper, an effectwhich may result from vertical air jets from the fluidizing plate beingundisturbed or uninterrupted for an extended period of time. With thethird embodiment, the powder intake portion extends through the bottomof the powder hopper, and the pump body for the powder ejectionapparatus is connected directly to the bottom of the powder hopper.

According to a fourth preferred embodiment of the invention, precisepowder flow control is achieved during pumping and during switching in amatter similar to the third preferred embodiment. Additionally, thefourth embodiment uses a reduced diameter stirring paddle locateddirectly above the powder intake portion of the apparatus. The powderintake, or inlet, extends through the bottom of the powder hopper andinto a pump body located therebelow. As with the third embodiment, inthe fourth embodiment powder is pumped via operation of an ejectornozzle directed toward an outlet line. Use of a transport air nozzle isoptional. During the off cycle, reverse air flow is used to preventpowder particles from flowing downwardly from the powder container intothe powder inlet of the apparatus. Additionally, the outlet line extendshorizontally from the pump body, without any upward turns, and connectsto a gun at its outlet end, which is directed vertically. With thisconstruction, powder does not fall back into the pump body due togravitational forces during the off cycle. Additionally, with thisconstruction interaction among the powder particles which producesaccumulation, adherence, agglomeration and/or static electricity fromfriction is reduced, thereby enhancing the uniformity of powder deliveryto an article to be coated and producing a more stable coating.

Optimally, the spray gun utilized in this fourth embodiment, and theother embodiments as well, has a straight, unobstructed flow path withan external charging electrode to prevent powder particles fromagglomerating within the spray gun.

These and other features of the invention will be more readilyunderstood in view of the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross sectional view of a powder ejection apparatusconstructed in accordance with a first preferred embodiment of theinvention.

FIG. 2 is a cross sectional view taken along lines 2--2 of FIG. 1.

FIG. 3 is a graph which depicts powder discharge rate for the powderejection apparatus shown in FIGS. 1 and 2.

FIG. 4 is a vertical cross sectional view of a powder ejection apparatusconstructed in accordance with a second preferred embodiment of theinvention.

FIG. 5 is a graph which illustrates operation of the ejector nozzle andthe reverse flow nozzle of the powder ejection apparatus shown in FIG.4.

FIG. 6 is a vertical cross section, similar to FIG. 4, of a variation ofthe second preferred embodiment of the invention.

FIG. 7 is a graph which illustrates operation of the ejection nozzle andthe reverse flow nozzle of the powder ejection apparatus shown in FIG.6.

FIG. 8 is a cross sectional schematic view of a powder coating systemconstructed in accordance with a third preferred embodiment of theinvention.

FIG. 9 is a graph which illustrates the operation of the powder coatingsystem shown in FIG. 8.

FIG. 10 is a cross sectional schematic view of a powder coating systemconstructed in accordance with a fourth preferred embodiment of theinvention.

FIG. 10A shows the powder coating gun of FIG. 10 in more detail.

FIG. 11 is a cross sectional view taken along lines 11--11 of FIG. 10.

FIG. 12 is a graph which illustrates the ON/OFF sequence for componentsof the powder coating system shown in FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-3 relate to a first preferred embodiment of the invention. Moreparticularly, FIG. 1 shows a powder ejection apparatus 10 which includesa pump body 12. The pump body 12 receives fluidized powder from a powderhopper 14 and pumps the powder through an ejection tube 17 connected tothe pump body 12. Directional arrow 18 shows the direction of flow ofthe powder. The tube 17 may be of any desired length to facilitatedirecting the powder flow to a spray gun. To enter pump body 12 from thepower hopper 14, the powder enters a powder inlet 20. This powder flowis caused by an ejector nozzle 22 mounted to pump body 12 and directedalong the same axis as tube 17. An air supply tube 24 is connected toejector nozzle 22, and as shown by directional arrow 25, supplies air ata relatively high pressure to ejector nozzle 22 to pump the powderoutwardly through tube 17. As ejector nozzle 22 directs the highpressure air stream toward tube 17, a low pressure is created at thepowder inlet portion 20, thereby causing the powder to move therethroughfrom the hopper 14 and into the pump body 12 to be pumped through tube17.

Because of the relatively small cross sectional dimension of inlet 20relative to powder hopper 14, powder has a tendency to adhere andaccumulate within the inlet 20 enroute to pump body 12, particularly ifthe powder has the property of high coherence, high adherence due toviscosity, low slipperiness, a tendency to agglomerate or low flowcharacteristics. If left unchecked, this powder accumulation will narrowthe powder inlet 20 and reduce the total volume of powder supplied tothe pump body 12 and to tube 17 per unit time, thereby adverselyaffecting the uniformity of the powder coating on the article beingcoated. This reduction in discharge quantity per unit time is shown inFIG. 3, designated by reference numeral 32.

To solve this problem, a pulsing air flow is directed from a pulsingnozzle 28 through the inlet 20 in a direction counter to that of thenormal powder flow inlet 20. Preferably, as shown in FIG. 2, the nozzle28 and the nozzle 22 are oriented such that their axes and flow paths donot intersect. Also, it is preferred that this reverse flow be sprayedat a pressure lower than the pressure of the air sprayed from nozzle 22.This pulsing air flow from nozzle 28 produces microvibrations in thepowder within the inlet 20, which prevents or reduces adherence andaccumulation of the powder along the sidewalls thereof. As a result, thequantity of powder transferred from the hopper 14 through the pump body12 and outwardly through tube 17 does not fluctuate with time duringoperation of the powder ejection apparatus 10. Therefore, the powderejection apparatus 10 can be operated continuously to produce uniformityin the volume of powder ejected per unit time, thereby assuringconsistent powder flow to a spray gun for spraying onto a surface to becoated and producing a higher quality coating.

EXAMPLE NO. 1

Applicant tested powder ejection apparatus 10 using the followingparameters:

Powder used--a fluoride powder flux;

Air pressure setting for the ejector nozzle 22--4 kg/cm² ;

Air pressure setting for nozzle 28--2 kg/cm², and the air wascontinuously pulsed to spray for a duration of about 20 to 100milliseconds and then to stop for a duration of about 40 to 200milliseconds.

Under these conditions, the powder ejection apparatus 10 ejected thefluoride powder in a relatively uniform quantity over the entire periodof operation, as shown in FIG. 3 by reference numeral 34, therebyimproving the powder flow uniformity.

While FIGS. 1 and 2 show nozzle 28 above and to the side of ejectornozzle 22, it is to be understood that nozzle 28 could be located in anyone of a number of different positions, so long as the oppositelydirected pulsing air flows through inlet 20 and into powder hopper 14.The primary consideration is that the two gas flows from nozzles 22 and28 should not cross. Also, the powder hopper 14 may be a fluidizing tankfor supporting the powder in a fluidized state, i.e., a fluidized bed.

FIGS. 4-7 relate to a second preferred embodiment of the invention. Moreparticularly, FIG. 4 shows a powder ejection apparatus and powdercoating system 110 which also provides uniformity in volume andconsistency for a powder coating, but in a slightly different manner.The apparatus 110 receives fluidized powder from a powder hopper 114 andpumps the powder to a spray nozzle 116 located at the end of an outlettube 117. The powder storage chamber 114 includes a fluiding plate 115located adjacent the bottom thereof through which air is directedupwardly, as shown by directional arrows 119, to fluidize the powderwithin hopper 114. Similar to the first embodiment, pressurized air issprayed from an ejector nozzle 122 aimed along the axis of the tube 117.However, in this second embodiment, instead of a single pump body 12,the powder ejection apparatus 110 has a pump body which includes aseparate intake portion 112a and an ejector portion 112b in fluidcommunication via a connector 112c. The intake portion 112a is in fluidcommunication with the powder hopper 114, thereby providing a flow pathfor fluidized powder from the powder hopper 114 to the ejector portion112b. More particularly, intake portion 112a includes powder inletportion 120 through which the powder must flow from powder hopper 114enroute to the ejector portion 112b. A reverse flow nozzle 128 mounts tointake portion 112a, and is directed toward powder inlet 120 to sprayair through the powder inlet 120 and into the powder hopper 114, counterto the normal flow direction of the powder.

Ejector nozzle 122 and reverse flow nozzle 128 are operatively connectedto a pressurized air source 125, via fluid lines 124 and 130,respectively. Fluid lines 124 and 130 each include an in line gasregulator, designated by reference numerals 134 and 140, respectively.Additionally, fluid lines 124 and 130 include solenoid valves 135 and138, respectively, which are electrically connected with an electriccontroller 142. The controller 142 controls the timing sequences of theoperation for the ejector nozzle 122 and the reverse flow nozzle 128 toalternately actuate the solenoid valves 135 and 138 to spray air fromnozzles 122 and 128, thereby alternating between drawing powder intopowder ejection apparatus 110 from hopper 114 and blowing powder awayfrom apparatus 110 into hopper 114.

Thus, compared to the first embodiment, which produced uniformity inpowder quantity per unit time during ejection, this embodiment achievesuniformity in ejection quantity per ON/OFF cycle. This is due to theuniformity in conditions during initiation of the "ON" portion of theON/OFF cycle of operation. At initiation, no powder resides in the inletportion 120, unlike prior systems which relied upon pinch valves andwere susceptible to deterioration with age and inadvertent powder flow.Preferably, the controller 140 is programmable to select the desiredoperating sequence.

During operation, when the ejector nozzle 122 sprays air through ejectorportion 112b, reduced pressure at inlet 120 and in connector tube 112ccauses the fluidized powder to flow from the powder hopper 114 throughpowder intake portion 120 and ejector portion 112b to nozzle 116. Thisproduces a spray pattern 118 of powder coating material from nozzle 116.After the nozzle 122 turns off, reverse flow nozzle 128 is turned on tospray air toward hopper 114, and in the opposite direction of the normalpowder ejection flow path of the fluidized powder. This spray fromnozzle 128 blows the powder out of the inlet portion 120 into thechamber 114. When nozzle 128 is turned off and nozzle 122 is turned on,the powder must move entirely from the chamber 114 to inlet 120, sinceno powder was already in the inlet 120 at the commencement of sprayingby nozzle 122. By cooperatively pulsing the ejector nozzle 122 and thereverse flow nozzle 128, the same quantity of powder can be ejectedduring each cycle of "ON/OF" sequences, thereby assuring uniformity inpowder ejection during coating operations which require switching andeliminating prior uniformity problems caused by inconsistent sprayvolumes especially during initiation of the "ON" portion of the cycle.This second embodiment of the invention is particularly suitable forspray coating articles carried a conveyor, due to the need for ON/OFFcycling at a coating station.

FIG. 5 shows the ON/OFF times for ejector nozzle 122 and reverse flownozzle 128, as designated by reference numerals 143 and 144,respectively. Reference 146 shows the duration of "ON" time for reverseflow air nozzle 128 and reference numeral 147 shows the duration of timethereafter until ejector nozzle 122 is turned back to "on" to commencepowder pumping. Thus, no powder is pumped during the time periodrepresented by the sum of 146 and 147.

With the reverse flow of air sprayed by nozzle 128 through powder inletportion 120, this invention eliminates the need to insert a pinch valve,or possibly a mechanical type valve, between powder hopper 114 and theinlet of ejector portion 112b for the purpose of starting and stoppingpowder flow.

FIG. 6 shows a variation of the second preferred embodiment of theinvention. According to this variation, the powder ejection apparatus110 includes a single piece pump body 112 to which the ejector nozzle122 and the reverse flow nozzle 128 are mounted. The ejector nozzle 122is aimed to spray air outwardly through line 117, and the reverse flownozzle 128 is aimed to spray air opposite the normal flow of thefluidized powder, through powder inlet 120 and toward powder hopper 114.

FIG. 7 shows timing control for switching the operation of ejectornozzle 122 and the reverse flow nozzle 128 for the apparatus 110 shownin FIG. 6. Because of the proximity of the nozzles 122 and 128, theswitching spray pulses between ON/OFF can be more closely timed toprovide a shorter ON/OFF cycle for the apparatus 110. Again, as with theapparatus shown in FIG. 4, the apparatus of FIG. 6 provides effectivestopping and starting of each powder pumping cycle, thereby assuringuniformity in the volume of powder pumped per cycle.

FIGS. 8 and 9 depict a third preferred embodiment of the invention whichcombines features from the first and the second embodiments. Morespecifically, FIG. 8 shows a powder coating system 210 for applyingelectrostatically charged particles to an article 209 to be coated. Theapparatus 210 conveys the particles through an outlet conduit 217 to aspray nozzle 216 which produces a spray pattern designated by referencenumeral 218. To electrostatically charge the particles, the apparatus210 includes a corona electrode 208 mounted adjacent the outlet 216. Thecorona electrode 208 is connected to a power supply (now shown) inelectrical controller 248 via an electrical cable 206.

The system 210 further includes a pump body 212 which receives powderfrom a powder hopper 214 and then conveys the powder to outlet conduit217. The hopper 214 includes a fluidizing plate 215, located at a bottomend thereof above a fluidizing air plenum 221. An air inlet 250 suppliespressurized air to the plenum 221, and a resulting upward flow of thepressurized air through the plate 215 causes the powder to be fluidized.A powder supply container 251 supplies powder to the hopper 214 inresponse to a level sensor (now shown) in a manner known in theindustry. The hopper 214 also includes a rotatable stirring blade 254which is driven by a motor 255. The stirring blade 254 reduceschanneling of the powder in the hopper 214, as described above.

Air to the inlet nozzle 250 is supplied via a line 258 connected to apressurized air supply source and controller, designated generally 240b,and the system 210 precisely controls the flow of air to inlet 250 via asolenoid valve 260, which is electrically controlled by an electricalcontroller 240a. The electrical controller 240a and the air supplysource and controller 240b cooperatively interact to control alloperations of the powder system 210. Preferably, each of the controllers240a and 240b includes a microprocessor to facilitate programmablecontrol.

The pump body 212 is connected to the bottom of the hopper 214 andincludes a powder inlet 220 which communicates with the hopper 214. Areverse flow nozzle 228 mounts to pump body 212 for directing a streamof air through the powder inlet 220 and toward the hopper 214, oppositethe normal flow direction of powder during pump operation. An ejectornozzle 222 also mounts to the pump body 212, and ejector nozzle 222delivers a high pressure air stream through the pump body 212 along theaxis of outlet line 217. Additionally, pump body 212 includes a gastransport nozzle 270 which supplies pressurized gas to an annular region271 within the pump body 212 and then toward outlet line 217 viachannels 272, which are also formed within the pump body 212. Thus, thetransport nozzle 270 supplies pressurized transport air downstream ofthe ejector air supplied by the ejector nozzle 222. As a result, powderpumped from pump body 212 and into line 217 is carried by an air streamconsisting of the confluence of the ejector air and the transport air.

Air transport nozzle 270 is supplied with pressurized air via a fluidline 273 connected to the controller 240b, and air flow is controlledvia a solenoid valve 274, which is electrically connected to controller240a. Similarly, ejector nozzle 222 is connected to controller 240b viaa fluid line 224, and ejector air flow is controlled via a solenoidvalve 235 which is electrically connected to electrical controller 240a.

In this embodiment, the reverse flow nozzle 228 includes two separatefluid supply lines 230a and 230b connected to controller 240b, throughwhich air flow is controlled by solenoid valves 238a and 238b,respectively. Downstream of the solenoid valves 238a and 238b, the lines230a and 230b merge to form a single air supply line 230 for nozzle 228.The use of two separate supply lines 230a and 230b and two separatesolenoid valves 238a and 238b allows the apparatus 210 to use a singlenozzle 228 for providing reverse pulsing during powder pumping, as inthe first embodiment, and reverse flow to stop powder from entering pumpinlet 220 when the pump 15 is idle, as in the second embodiment.

In operation, at start-up, solenoid valves 235, 274 and 238b are closed.Solenoid valves 260 and 238a are opened. This produces fluidizing airflow into fluidizing plenum 221 via hopper inlet 250 to fluidizeparticles in the hopper 214. Additionally, the reverse flow throughnozzle 228 prevents powder from entering the powder inlet 220. Thepressure of the air supply sprayed from nozzle 228 may initially need tobe adjusted to accomplish this objective by means of a regulator (nowshown). Additionally, the stirring blade 254 is rotated by means ofmotor 255. Solenoid valve 260 remains open so that powder iscontinuously fluidized during the operation of system 210, and likewiseblades 254 are at all time continuously rotating wherever the system 210is in operation. To spray powder, valves 235 and 274 are opened, andsolenoid valve 238a is closed. This causes nozzle 222 and nozzle 270 tospray ejector air and transport air, respectively, thereby causingpowder to be pumped from hopper 214 through pump body 212 to outlet line217.

At the same time, solenoid valve 238b is opened and closedintermittently (pulsed) based on an electrical signal from thecontroller 240a. Preferably, during this intermittent opening andclosing of valve 238b, the air sprayed into pump body 212 from nozzle228 is adjusted to a pressure lower than the pressure of the pressurizedair sprayed from the ejector nozzle 222. Valve 238b continues to cyclefor so long as solenoid valves 235 and 274 are open. With the ejectornozzle 222 spraying air, due to opening of solenoid valve 235, powderfrom the hopper 214 is drawn into pump body 212 through powder inlet 220via venturi operation. Additionally, due to the opening of solenoidvalve 274, transport nozzle 270 supplies pressurized transport air tothe pump body 212 downstream of ejector nozzle 222. As a result, powderparticles are pumped through line 217 and are sprayed outwardly fromnozzle 216, whereupon corona electrode 208, powdered by cable 206 viapower supply and controller 204a, electrostatically charges theparticles to render them electrostatically attracted to the article 209to be coated. Article 209 is typically electrically grounded by theconveyor.

To temporarily stop the flow of powder from the nozzle 216, valves, 235,274 and 238b are closed, and at the same time, valve 238a is opened.This causes air flow from nozzle 228 to go from the intermittent pulsingoperation at relatively low pressure to reverse flow operation whereinan air stream is sprayed at sufficient pressure to force the powderparticles in the powder inlet 220 back into the hopper 214. Thisoperation of the solenoid valves is shown more clearly in FIG. 9, withreference numeral 280 indicating a temporary stoppage of powder flow.

With this powder coating system 210, a very consistent powder coatingmay be applied to an article 209, with a uniformity of powder thicknessand high quality assured, due to improved control of the volume ofpowder delivered per unit time during the "ON" portion of the ON/OFFcycle of operation and the ability to temporarily halt flow of thepowder from the hopper 214 during switching between "ON" and "OFF". Thisassures uniformity in conditions each time the powder pump of system 210is switched "ON".

EXAMPLE 2

A high quality coating was achieved with powder coating system 210 usingthe following parameters:

Powder--fluoride powder flux;

Rotation frequency of stirring blade--60 rpm;

Pressurized air for fluidization inlet 250--2 kg/cm² ;

Pressurized air for ejector nozzle 222--4 kg/cm² ;

Pulsed air flow for nozzle 228/solenoid valve 238b--2 kg/cm² with pulsed"ON" time of about 20-100 milliseconds and "OFF" time of about 40-200milliseconds;

Reverse flow air for nozzle 228/solenoid valve 238a--3 kg/cm² ;

Pressurized air for transport Nozzle 270--2-3 kg/cm².

FIGS. 10-12 relate to a fourth preferred embodiment of the invention,which varies somewhat from the third preferred embodiment, but stillprovides reverse pulsing during the pumping of powder and reverse flowwhen the pump is idle. FIG. 10 shows a powder coating system 310 whichsprays powder in a spray pattern 318 onto an article to be coated 309.If desired, the article 309 may be carried on a conveyor 301 and/orcoated within an environmentally controlled enclosure 302 whereinoversprayed powder would be collected and possibly returned to hopper314. The apparatus 310 conveys fluidized powder from hopper 314, througha pump body 312, through an outlet line 317 to an electrostatic spraygun 316, such as a Model NPE-4AH, available from Nordson Corporation,Amherst, Ohio, and shown in U.S. Pat. No. 4,630,777, which is herebyincorporated by reference in its entirety. The hopper 314 includes afluidizing plate 315 above an air plenum 321. Air supplied into theplenum 321 via inlet 350 passes upwardly through the plate 315 tofluidize the powder particles within hopper 314. A paddle 354 is mountedwithin hopper 314 and rotated via motor 355 to uniformly mix the powderparticles.

Particles are drawn from the hopper 314 into the pump body 312 viaventuri action caused by operation of an ejector nozzle 322 mounted tothe pump body 312 and directed at outlet line 317. To get to the pumpbody 312, the particles move through a powder inlet 320, which ispreferably located directly below rotating paddle 354. As shown in FIG.11, pump body 312 further includes a pulse nozzle 328 directed at thehopper 314 and along powder inlet 320, and a flow nozzle 329 which isalso directed at the hopper 314 along the powder inlet 320. Thus, thisfourth preferred embodiment of the invention differs from the thirdpreferred embodiment in that separate nozzles, i.e., nozzles 328 and329, are used for the separate functions of reverse pulsing inlet 320during pumping, and reverse flow to prevent the particles from enteringinlet 320 when the pump is idle, respectively. This is in contrast tothe single nozzle 228 which was used with two separate solenoid valves238a and 238b and fluid lines 230a and 230b in the third preferredembodiment for these same two functions.

FIG. 10 also clearly shows outlet line 317 extending horizontally frompump body 312, without any horizontal or vertical bends, except for thedownward bend before gun 316, and is located either level with orentirely below the outlet of pump body 312. Also, spray gun 316 isoriented vertically and located below the outlet of pump body 312. Thisstructure eliminates the possibility of powder particles returning tothe pump body 312 under gravitational forces when the ejector nozzle 322is switched off. The gun 316 is also oriented vertically to furtherreduce the possibility of collisions and/or coherence of the powderparticles during flow from pump body 312 to the spray nozzle of gun 316.In total, the flow path makes only one turn between pump body 312 andthe article 309.

FIG. 11 shows the relative positions of the nozzles 328 and 329 withrespect to nozzle 322. Use of the two separate nozzles, 328 and 329, forthe separate functions of pulsing and stoppage, respectively, producesmore precision in these controls and further allows the flows emanatingfrom these nozzles to be oriented in a manner which does not interferewith the spray from nozzle 322.

FIG. 12 shows the ON/OFF timing operation of the sprays from nozzles322, 328 and 329. Air flow from these nozzles is controlled in the samemanner as described with respect to the third preferred embodiment.

FIG. 10A shows the spray gun 316 in greater detail. Spray gun 316 has apowder flow conduit 370 having an inlet 351 at one end and a spraynozzle 352 at the other end. Preferably, spray nozzle 352 is a slotnozzle having a 0.12 inch wide slot available from Nordson Corporation,Amherst, Ohio as part number 117,158. Conduit 370 is completelyunobstructed up to nozzle 352 which defines spray pattern 318. This isfacilitated by the use of a charging electrode 360 which is completelyexternal to conduit 370 and nozzle 352. With this spray gun design thereare no places within the gun for the powder particles to agglomerate,and this facilitates the consistent, uniform application of powder toarticles by means of the system 310 of this fourth embodiment.

In view of the above detailed description of four preferred embodiments,it will be understood that variations will occur in employing theprinciples of this invention, depending upon materials and conditions,as will be understood to those of ordinary skill in the art.

We claim:
 1. A powder coating system comprising:a powder hoppercontaining fluidized powder particles; a pump body having an inlet andan outlet; an ejector nozzle mounted to the pump body and aimed at theoutlet; means for supplying fluidized powder particles from the powderhopper to the inlet of the pump body, the ejector nozzle adapted tospray pressurized air toward the outlet to transport the powderparticles along a flow path which extends from the powder hopper intothe pump body via the inlet and then out of the pump body via theoutlet; a second nozzle mounted to the pump body and aimed at the inlet,the second nozzle adapted to spray pulses of air at the inlet in adirection opposite the flow of the powder particles along the flow pathcaused by the ejector nozzle; and a powder applicator device forreceiving powder particles from said outlet and spraying the powderparticles onto an article to be coated.
 2. The powder coating system ofclaim 1 wherein the ejector nozzle is aligned along a first axis and thesecond nozzle is aligned along a second axis and the ejector nozzle andthe second nozzle are mounted so that the first axis and the second axisdo not intersect.
 3. The powder coating system of claim 2 wherein theejector nozzle and the second nozzle are oriented substantiallyperpendicularly to each other.
 4. The powder coating system of claim 1wherein the air pressure of the ejector nozzle is sufficiently higherthan the air pressure of the second nozzle so as to not disrupt thetransport of powder particles along the flow path caused by the ejectornozzle.
 5. The powder coating system of claim 1 wherein the powderapplicator device comprises a spray gun having an external chargingelectrode.
 6. The powder coating system of claim 5 wherein the spray gunhas an inlet and a spray nozzle and a straight, unobstructed powder flowpassage between the inlet and the spray nozzle.
 7. A powder coatingsystem comprising:a powder hopper containing fluidized powder particles;a pump body having an inlet and an outlet; an ejector nozzle mounted tothe pump body and aimed at the outlet; means for supplying fluidizedpowder particles from the powder hopper to the inlet of the pump body,the ejector nozzle adapted to spray pressurized air toward the outlet totransport the powder particles along a flow path which extends from thepowder hopper into the pump body via the inlet and out of the pump bodyvia the outlet, during an "ON" portion of an ON/OFF cycle of operation;a second nozzle mounted to the pump body and aimed at the inlet, thesecond nozzle adapted to spray, during an "OFF" portion of the ON/OFFcycle of operation, a flow of gas toward the inlet in a direction whichis reverse with respect to the flow of powder particles along the flowpath caused by the ejector nozzle, the reverse flow blowing powderparticles in the inlet back towards the powder hopper; and a powderapplicator device for receiving powder particles from said outlet andspraying the powder particles onto an article to be coated.
 8. Thepowder coating system of claim 7, further comprising:a controller forcontrolling the ejector nozzle and the second nozzle to coordinateswitching between the "ON" and "OFF" portions of the cycle of operation.9. The powder coating system of claim 7 wherein the ejector nozzle isaligned along a first axis and the second nozzle is aligned along asecond axis and the ejector nozzle and the second nozzle are oriented sothat the first and second axes do not intersect.
 10. The powder coatingsystem of claim 8 wherein the pump body further comprises;an outletportion to which the ejector pump is mounted; an intake portion to whichthe second nozzle is mounted; and a connector interconnecting the outletportion and the intake portion.
 11. The powder coating system of claim 5wherein the ejector nozzle pressure is greater than the second nozzlepressure.
 12. The powder coating system of claim 7 wherein theapplicator device comprises a spray gun having an external chargingelectrode.
 13. The powder coating system of claim 12 wherein the spraygun has an inlet and a spray nozzle and a straight, unobstructed powderflow passage between the inlet and the spray nozzle.
 14. A powdercoating system comprising;a powder hopper; means for maintaining powderparticles in the powder hopper in a fluidized state; a pump body incommunication with the powder hopper, including an inlet therebetween;an outlet line connected to the pump body wherein the powder hopper, thepump body, the inlet and the outlet line define a flow path for powderparticles pumped from the hopper; an ejector nozzle mounted to the pumpbody and aimed at the outlet line, the ejector nozzle adapted to sprayair through the pump body toward the outlet during an "ON" portion of an"ON/OFF" cycle of operation, thereby to draw powder particles from thepowder hopper into the pump body and to pump the powder particles out ofthe pump body through the outlet line; means for pulsing an air flowtoward the inlet, during the "ON" portion of the cycle of operation;means for directing a reverse airflow toward the inlet, during an "OFF"portion of the "ON/OFF" cycle; and a powder applicator device forreceiving powder particles from the outlet line and spraying the powderparticles onto an article to be coated.
 15. The powder coating system ofclaim 14, further comprising:a single nozzle adapted to spray both thepulsing flow and the reverse flow.
 16. The powder coating system ofclaim 8, further comprising:a transport nozzle mounted to the pump bodyalong the flow path downstream from the ejector nozzle, the powderparticles carried out the outlet line during the "ON" portion by airfrom the ejector nozzle and the transport nozzle.
 17. The powder coatingsystem of claim 14, further comprising:a controller operativelyconnected to the ejector nozzle, the means for pulsing and the means fordirecting a reverse air flow and adapted to control the operationthereof during the "ON" and "OFF" portions of the cycle.
 18. The powdercoating system of claim 14, further comprising:a spray gun located at anend of the outlet line, the outlet line extending horizontally from thepump body and the gun located below the outlet line.
 19. The powdercoating system of claim 14 wherein the inlet is located below the powderhopper and further comprising:a rotatable stirring member located in thepowder hopper above the inlet.
 20. The powder coating system of claim14, further comprising:means for electrostatically charging the powderparticles as the powder particles exit the outlet line toward an articleto be coated.
 21. The powder coating system of claim 14 wherein themeans for pulsing further comprises a first nozzle directed toward theinlet and the means for directing a reverse flow comprises a secondnozzle directed toward the inlet.
 22. The powder coating system of claim14 wherein the applicator device comprises a spray gun having anexternal charging electrode.
 23. The powder coating system of claim 22wherein the spray gun has an inlet and a spray nozzle and a straight,unobstructed powder flow passage between the inlet and the spray nozzle.24. A powder coating method comprising the steps of:fluidizing powderparticles within a powder hopper; providing a pump body in fluidcommunication with the powder hopper; spraying air from an ejectornozzle toward an outlet of the pump body, thereby causing a flow ofpowder particles to move from the powder hopper through an inlet to thepump body and to be pumped outwardly from the outlet in a firstdirection, along a flow path; pulsing air from a second nozzle towardthe inlet, the pulsed air flowing in a direction opposite to the flow ofpowder particles along the flow path caused by the ejector nozzle;delivering powder particles from the outlet to a powder applicatordevice; and spraying powder particles from the powder applicator deviceonto an article to be coated.
 25. The method of claim 24 wherein the airsprayed from the ejector nozzle is sprayed at a first pressure and theair pulsed from the second nozzle is pulsed at a second pressure, andthe second pressure is less than the first pressure.
 26. The method ofclaim 24 wherein the ejector nozzle is aligned along a first axis andthe second nozzle is aligned along a second axis and the ejector nozzleand the second nozzle are aimed so that the first axis and the secondaxis do not intersect.
 27. A powder coating method comprising the stepsof:fluidizing powder particles within a powder hopper; providing a pumpbody in fluid communication with the powder hopper; spraying air from anejector nozzle toward an outlet of the pump body during an "ON" portionof an "ON/OFF" cycle of operation, thereby pumping a flow of powderparticles from the powder hopper through an inlet to the pump body andout the outlet in a first direction, along a flow path; and spraying airfrom a second nozzle toward the inlet in a direction opposite to theflow of powder particles along the flow path caused by the ejectornozzle, during an "OFF" portion of the "ON/OFF" cycle of operation;delivering powder particles from the outlet to a powder applicatordevice for coating an article to be coated.
 28. The powder coatingmethod of claim 27, further comprising the step of:spraying transportair into the flow path during the "ON" portion of the cycle ofoperation, the transport air being added to the flow path downstream ofthe spray from the ejector nozzle.
 29. The powder coating method ofclaim 27, further comprising the step of:pulsing air toward the inlet ina direction opposite to the flow of powder particles along the flow pathcaused by the ejector nozzle during the "ON" portion of the cycle ofoperation.
 30. The powder coating method of claim 29 wherein the pulsingair is also sprayed from the second nozzle.
 31. The powder coatingmethod of claim 29 wherein the pulsing air is sprayed from a thirdnozzle.
 32. The powder coating method of claim 29 wherein the pulsingair is sprayed from a pulse nozzle, and further comprising the stepof:controlling operation of the ejector nozzle, the second nozzle andthe pulse nozzle by a controller which simultaneously sprays air fromthe ejector nozzle and the pulse nozzle during the "ON" portion of thecycle and then terminates the air flow from both nozzles, and sprays airfrom the second nozzle during the "OFF" portion of the cycle.
 33. Thepowder coating method of claim 27 and further comprising the stepof:stirring the powder particles with a rotating member while in thepowder hopper.
 34. The powder coating method of claim 27 wherein theoutlet further comprises an outlet line connected to the powderapplicator device, the powder applicator device located below theoutlet.
 35. The powder coating method of claim 34 wherein the powderapplicator device is vertically oriented with respect to the outletline.
 36. The powder coating method of claim 34 wherein the outlet lineextends substantially horizontally from the pump body.
 37. The powdercoating method of claim 27 further comprising the step ofelectrostatically charging the powder particles by means of theapplicator device.
 38. The powder coating method of claim 37 whereinduring the electrostatic charging step the powder particles are chargedby an electrode positioned outside of the flow path of the powderapplicator device.
 39. The powder coating method of claim 38 wherein thepowder particles are passed through a powder flow passage of theapplicator device which is straight and unobstructed between the inletto the device and the spray nozzle on the device.
 40. A powder coatingsystem comprising:a powder hopper containing fluidized powder particles;a pump body having an inlet and an outlet, wherein the inlet of the pumpbody opens to the powder hopper to place the pump body and the powderhopper in fluid communication; an ejector nozzle mounted to the pumpbody and aimed at the outlet, the ejector nozzle adapted to spraypressurized air toward the outlet to transport the powder particlesalong a flow path which extends from the powder hopper into the pumpbody via the inlet and then out of the pump body via the outlet; asecond nozzle mounted to the pump body and aimed at the inlet, thesecond nozzle adapted to spray pulses of air at the inlet in a directionopposite the flow of the powder particles along the flow path caused bythe ejector nozzle; and a powder applicator device for receiving powderparticles from said outlet and spraying the powder particles onto anarticle to be coated.
 41. The powder coating system of claim 40 whereinthe powder hopper has walls and a bottom fluidizing plate, and thepowder particles contained in the powder hopper reside within aninternal volume defined by the walls and the fluidizing plate, whereinthe inlet of the pump body does not protrude into the internal volume ofthe powder hopper.
 42. The powder coating system of claim 40 wherein thepowder hopper has a bottom and the inlet of the pump body opens to thebottom of the powder hopper.
 43. The powder coating system of claim 40and further comprising a fluidizing plate located in the powder hopperwhich defines a fluidizing air plenum below the fluidizing plate, thepowder particles in the powder hopper located above the fluidizingplate, wherein the inlet to the pump body extends through the fluidizingair plenum and terminates at the fluidizing plate, the pump body locatedbelow the powder hopper.
 44. The powder system of claim 43 and furthercomprising:a rotatable stirring member located in the hopper above theinlet.